The present invention relates to novel epoxy acrylates of higher molecular weight and to novel polymolecular carboxyl group-containing epoxy acrylates, to the preparation thereof, to the use of said acrylates in photoresist formulations, and to the use of said formulations, in particular in the field of printed wiring boards and printing plates.
Epoxy acrylates are known in abundance and are also used, inter alia, in compositions used as photoresist formulations, for example in GB 2 175 908, in which resins derived from an unsaturated polybasic acid anhydride and a reaction product of an epoxy acrylate and an unsaturated monocarboxylic acid are used.
Formulations for solder resists that contain reaction products of epoxy novolak resins with acrylic acid and cyclic carboxylic anhydrides are disclosed, inter alia, in EP 0 273 729. They are developable in aqueous alkaline media and have good thermal resistance and photosensitivity. Their resistance to chemicals, however, is unsatisfactory.
EP 0 418 011 discloses compositions for solder masks that are likewise based on reaction products of epoxy cresol novolaks with acrylic acid and cyclic dicarboxylic anhydrides, using 0.4 to 0.9 equivalent of acrylic acid per equivalent of epoxy group, such that the final product simultaneously contains acid and epoxy groups in the same molecule. A second thermal crosslinking reaction between these two functionalities is thereby made possible in the application of these resist compositions. The problem here is, however, aside from the preparation of the products (danger of gelation in the reaction with the anhydride), the storage stability, as the formulation containing such reaction products has a certain reactivity even at room temperature.
All these cited epoxy acrylates are quite generally relatively low-molecular.
Photochemically or thermally cured epoxy acrylates that are derived from low molecular epoxy resins and epoxy novolaks are known for their good thermal and mechanical properties as well as for their good resistance to aggressive chemicals. However, the tackiness and edge coverage of the resist films obtained with these systems on conductors owing to the fairly low relative molar mass are unsatisfactory. In practical application it is therefore often necessary to avoid these shortcomings by adding highly polymerised polymer binders. Such binders normally contain no functional acrylate groups and do not react concurrently during the photochemical or thermal cure, i.e. they are not incorporated as xe2x80x9cpassivexe2x80x9d components in the network and therefore result in a dilution of the network density, which, in turn, adversely affects in particular the resistance to chemicals and the electrical properties of processed resist layers. Furthermore, the photosensitivity decreases as a consequence of the xe2x80x9cdilutionxe2x80x9d of the acrylate groups. The addition of highly polymerised polymer binders induces high viscosity of these formulations even if the solids content is relatively low and therefore often results in serious problems in coating.
Japanese patent Kokai Hei 04-294352 discloses the modification of epoxy-novolak resins by reaction with an unsaturated monocarboxylic acid and subsequently with an unsaturated anhydride of a polycarboxylic acid and the use thereof in photosensitive aqueous formulations.
It is therefore the object of this invention to provide acrylates that do not have the shortcomings referred to above.
This object is achieved in the practice of this invention by novel epoxy acrylates and novel carboxyl group-containing epoxy acrylates of higher molecular weight which, when used in resist formulations, are able to function without or with only minor amounts ( less than 10% by weight) of additional polymer binders. They are obtained by reaction of so-called xe2x80x9cadvancedxe2x80x9d epoxy resins with, typically, (meth)acrylic acid.
Specifically, the invention provides novel epoxy acrylates of formula II 
wherein
M is the group of formula 
R1 is xe2x80x94H or xe2x80x94CH3, R2 xe2x80x94H, xe2x80x94CH3 or phenyl,
R is C1-C4 alkyl or halogen, preferably CH3 or Br,
x is an integer from 0 to 3, preferably 0 or 1, and
Y is a linking group of formula 
wherein R3 and R4 are each independently of the other hydrogen or C1-C4alkyl, or R3 and R4, together with the linking carbon atom, form a 5- or 6-membered hydrocarbon ring, and the aromatic radicals of the linking group Y are unsubstituted or substituted by halogen or C1-C4alkyl,
X is xe2x80x94Sxe2x80x94, xe2x80x94Oxe2x80x94, or xe2x80x94SO2xe2x80x94, and
n is an integer from 0 to 300, preferably from 0 to 30, with the proviso that at least 10 mol % of the radicals M have the structure of formula 
in which R1 and R2 are as defined above.
If the aromatic radicals of the linking group Y are substituted, then suitable halogen substituents are fluoro, chloro and, preferably, bromo; and suitable C1-C4alkyl substituents are straight-chain or branched C1-C4alkyl groups such as methyl, ethyl, n- and isopropyl, n-, sec- and tert-butyl.
Particularly preferred linking groups Y have the formula 
wherein R3 and R4 are as defined above and, in particular, have the formulae 
Preferred epoxy acrylates have the formula II, wherein R is xe2x80x94H (for xxe2x95x90O) or xe2x80x94CH3, R1 is xe2x80x94H or xe2x80x94CH3, R2 is xe2x80x94H, x is 0 or 1, n is an integer from 0 to 30, and Y is a linking group of formula 
wherein R3 and R4 are xe2x80x94H or C1-C4alkyl and the aromatic radicals of the linking group are unsubstituted or substituted by halogen or C1-C4alkyl.
The novel epoxy acrylates of formula II may be obtained by reacting an advanced epoxy novolak of formula I 
wherein R, Y, n and x are as defined for formula II, with an ethylenically unsaturated monocarboxylic acid, if necessary in the presence of a catalyst and of a polymerisation inhibitor, at elevated temperature.
The advanced epoxy novolaks of formula I are known (q.v. inter alia JP patent Kokai Hei 1-195056) and are prepared from a bisphenol of formula HOxe2x80x94Yxe2x80x94OH, wherein Y is as defined above, and an epoxy novolak, in known manner, the molar amount of bisphenol being conveniently 0.01 to 0.1 mol per epoxy group in the epoxy novolak.
These advanced epoxy novolaks of formula I carry secondary aliphatic hydroxyl groups.
The reaction of the advanced epoxy novolaks of formula I to give the novel epoxy acrylates of formula II is carried out in known manner, conveniently by reaction with an ethylenically unsaturated monocarboxylic acid of formula 
Suitable acids are crotonic acid, cinnamic acid and, preferably, acrylic acid or methacrylic acid or a mixture thereof. R1 and R2 have the meanings given above.
It is preferred to use a catalyst in this reaction. Particularly suitable catalysts are metal salts such as chromium compounds, amines such as triethylamine or benzyldimethyl-amine, also ammonium salts such as benzyltrimethylammonium chloride, or also triphenylphosphine and triphenylbismuth.
A solvent may be added to the reaction, as the advanced epoxy resins of formula I are in the form of solids. However, the solvent must be inert to the educt. Suitable solvents include: ketones such as acetone, methyl ethyl ketone, cyclohexanone; esters such as ethyl and butyl acetate, ethoxyethyl acetate or methoxypropyl acetate; ethers such as dimethoxyethane and dioxane; aromatic hydrocarbons such as toluene, benzene and xylenes, as well as mixtures of two or more of these solvents.
The temperature will conveniently be in the range from 80 to 140xc2x0 C., the reaction with acrylic acid being preferably carried out in the range from 80 to 120xc2x0 C. and that with methacrylic acid preferably in the range from 80 to 140xc2x0 C.
A polymerisation inhibitor may also be added to the reaction medium, suitably hydroquinone, hydroquinone monomethyl ether and 2,6-di-tert-butyl-p-cresol.
It is desirable to introduce air or a mixture of nitrogen/oxygen into the reaction medium, as some of the aforementioned polymerisation inhibitors are effective only in the presence of oxygen. Depending on the amount of ethylenically unsaturated monocarboxylic acid used, epoxy acrylates of formula II that are completely or only partially acrylated are obtained. The monocarboxylic acid can be used in equimolar amounts with respect to the epoxy groups or in less than equivalent amount. The completely reacted epoxy acrylates contain almost no more epoxy groups.
The novel epoxy acrylates of formula II usually need neither be isolated from the reaction medium nor purified. The reaction solution can be used direct as obtained in the synthesis.
The partially as well as the completely reacted products of formula II contain aliphatic hydroxyl groups originating from the reaction of the epoxy groups with the ethylenically unsaturated monocarboxylic acid. They may additionally contain aliphatic hydroxyl groups from the educt.
The completely acrylated epoxy acrylates of formula II can then be further reacted to carboxyl group-containing epoxy acrylates of formula III 
wherein
A is hydrogen or the group of formula 
and
R1, R2, R, n, x and Y are as defined above for formula II and R5 is the radical of a cyclic anhydride of a polycarboxylic acid after removal of the anhydride radical, and at least 10 mol % of the radicals A, as in formula II, have the structure of formula 
Preferred epoxy acrylates of formula III correspond to the preferred epoxy acrylates of formula II as indicated above, and the radicals R1, R2, R, x, n and Y in formula III have the same preferred meanings as given in connection with formula II.
As the completely reacted epoxy acrylates of formula II contain almost no more epoxy groups, they can be reacted with cyclic anhydrides of polycarboxylic acids. In this case, the aliphatic hydroxyl groups (formula II) react with the cyclic anhydride to effect ring opening and hemiester formation. In this reaction, for each reacted hydroxyl group a carboxylic acid bonded to the resin forms. The reaction comprises reacting the epoxy acrylate of formula II with the cyclic anhydride, in the absence or presence of a catalyst and of a polymerisation inhibitor, at elevated temperature. The OH groups of the compounds of formula II are completely or partially acylated, accompanied by ring opening of the anhydride. It is therefore advantageous that the epoxy acrylates of formula II contain no more epoxy groups, otherwise gelation occurs. The reaction is known per se.
Suitable cyclic anhydrides of polycarboxylic acids typically include succinic anhydride, maleic anhydride, glutaric anhydride, tetrahydrophthalic anhydride, itaconic anhydride, phthalic anhydride, hexahydrophthalic anhydride, 3-methyl- and 4-methylhexahydro-phthalic anhydride, 3-ethyl- and 4-ethylhexahydrophthalic anhydride, 3-methyl-, 3-ethyl-, 4-methyl- and 4-ethyltetrahydrophthalic anhydride, and trimellitic anhydride.
Preferred anhydrides are succinic, tetrahydrophthalic, hexahydrophthalic and phthalic anhydride.
Suitable catalysts typically include amines such as triethylamine, benzyldimethylamine, pyridine or dimethylaminopyridine, or triphenylphosphine or metal salts such as chromium or zirconium compounds.
If desired, a solvent may be added to the reaction medium, as the epoxy acrylates of formula II are in the form of solids. The solvent must, however, be inert to the cyclic anhydride, so that hydroxyl group-containing solvents are not suitable. The solvents cited in connection with the reaction with the ethylenically unsaturated monocarboxylic acids may suitably be used, provided they contain no functional groups that react with anhydrides.
The reaction temperature is conveniently in the range from 60 to 140xc2x0 C., and suitable polymerisation inhibitors are typically hydroquinone, hydroquinone monomethyl ether and 2,6-di-tert-butyl-p-cresol.
It is desirable to introduce dry air or a mixture of nitrogen/oxygen into the reaction medium. In a preferred embodiment of the invention, the epoxy acrylates of formula II are further reacted, without isolation, in the same reactor to the derivatives of formula III modified with carboxyl groups.
Isolation and purification of the novel carboxyl group-containing epoxy acrylates of formula III is usually not necessary. The reaction solution can be further used as obtained in the synthesis.
Owing to the unsaturated groups present in the molecule, the epoxy acrylates of formula II and the carboxyl group-containing epoxy acrylates of formula III are thermally and photo-chemically crosslinkable. They can therefore be used and applied as acrylate components in photoresist formulations for the production of solder resists or primary resists by known methods, as for example disclosed in Swiss patent application 2005/93-4, filed on Jul. 2nd 1993, entitled xe2x80x9cPhotopolymerisable compositionsxe2x80x9d, and give resist layers having enhanced thermal, mechanical, electrical and chemical properties. The resist formulations prepared therefrom are used in particular in the field of printed wiring boards as solder resists or primary resists (etch resist or galvanoresist), and of printing plates. Suitable developers are aqueous as well as aqueous-organic or organic systems. Owing to the presence of carboxyl groups in the compounds of formula III, these systems are particularly suitable for the preparation of aqueous-alkaline developable photoresists.
Compared with low molecular epoxy acrylates in formulations that contain polymer binders, it is surprising that formulations containing epoxy acrylates of higher molecular weight without the addition of such polymer binders bring about an enhancement and not a loss of photosensitivity, and also that no increase in tackiness results. Furthermore, use of the formulations as solder resists results in improved edge coverage of the conductors. As no additional polymer binders are used in such formulations, further advantages accrue with respect to the thermal, mechanical and electrical properties and, in particular, to the resistance to chemicals of the resist compositions prepared therefrom. The novel epoxy acrylates of formula II and the carboxyl group-containing epoxy acrylates of formula III have an increased glass transition temperature.